1. Field of the Invention
This invention relates to a nanoparticle, more particularly to a superparamagnetic nanoparticle suitable for use in magnetic resonance imaging (MRI) diagnosis.
2. Description of the Related Art
Nanoparticles, especially magnetic nanoparticles, may have a contrast enhancing effect for MRI diagnosis under a magnetic field. For example, ultrasmall superparamagnetic iron oxide nanoparticles (USPIO) can considerably reduce the spin-spin relaxation time (T2) of protons of the targeted tissue under a magnetic field, thereby enhancing the contrast of a MRI image. Hence, USPIO has been widely used as a MRI contrast agent. In addition, USPIO can be also incorporated into magnetic field-guided drug delivery vehicles for cancer treatment.
However, when USPIO is used in vivo, the problems of self-aggregation and adsorption to plasma albumin of USPIO tend to occur due to the high specific surface area thereof. As a consequence, USPIO may be undesirably consumed by phagocytosis for macrophages and cannot be coupled to the targeted tissue.
Taiwanese patent publication No. 200808815 discloses a MRI contrast agent that has the following formula and that is prepared by bonding folic acid to the modified USPIO. The contrast agent in this publication was used to detect position of the cancer cell through targeting to the folic acid receptors on the cancer cell membrane.

Although there have been researches on modifying the surface of USPIO to avoid self-aggregation and adsorption and on the use of targeting ligand in the nanoparticles to achieve a highly specific targeting to cancer cells, there is still a need in the art to find nanoparticles suitable for use as a contrast agent that has a greater effect on reducing T2 so as to accomplish higher sensitivity and good imaging effect.